1. Field of Invention
The field of invention relates generally to lasers and, more specifically but not exclusively, relates to a surface emitting laser with an integrated absorber.
2. Background Information
Semiconductor lasers have a variety of applications including communication systems and consumer electronics. Generally, semiconductor lasers may be categorized as edge-emitting lasers or surface emitting lasers (SELs). Edge-emitting lasers emit radiation parallel to the semiconductor wafer surface while SELs emit radiation perpendicular to the semiconductor wafer surface. Excitation of the gain region of semiconductor lasers may be through optical pumping or electrical pumping.
Two common types of SELs are vertical cavity surface emitting laser (VCSEL) and vertical external cavity surface emitting laser (VECSEL). Referring to FIG. 1A, a VCSEL 100 is shown. A gain region 106 is sandwiched between mirror 104 and mirror 108. Such mirrors include distributed Bragg reflector (DBR) mirrors. Mirrors 104 and 108 define a laser cavity 112. Laser output 110 is emitted from the mirror 108 perpendicular to the gain region 106.
FIG. 1B shows a VECSEL 150. Mirror 158 is mounted externally and positioned above gain region 156. Mirror 154 and 158 define a laser cavity 162. Mirror 154 includes a DBR mirror. Laser output 160 is emitted from mirror 158.
Mode-locked lasers are used to generate narrow optical pulses on a time scale of picoseconds or less. In general, mode locking involves aligning the phases of longitudinal modes of the laser resulting in a periodic train of short pulses in the laser output. FIG. 1C shows a graph 165 of optical power versus time of a mode-locked laser. The repetition rate of the laser output is based on the period between pulses in graph 165. Mode locking may be achieved through active mode-locking or passive mode-locking. Active mode-locking uses frequency modulation or amplitude modulation through externally controlled modulators. Passive mode-locking is achieved through an absorber, which may include a saturable absorber material. The saturable absorber material may be fabricated from semiconductor material. The saturable absorber material may be fixed to a mirror, which may include a DBR mirror, to form a semiconductor saturable absorber mirror (SESAM).
In a passively mode-locked laser, the desired laser output of short pulses is provided via the absorber. The effect of a saturable absorber in a laser cavity is to favor parts of the circulating radiation with higher intensity over those with lower intensity. After many round-trips, this often leads to the formation of a single short pulse circulating in the cavity. This mechanism is called mode locking because in the frequency domain it corresponds to the creation of a fixed phase relationship between the longitudinal modes of the cavity. The circulating pulse in the laser cavity generates one output pulse each time it hits the output coupler. Thus, a regular pulse train is produced.
FIG. 1D shows a VECSEL 170 with a non-integrated absorber. VECSEL 170 includes a gain region 174 layered on a mirror 172. An output coupler 176 is positioned above the gain region 174. Mirror 172 and output coupler 176 define a laser cavity 178. An optical pump 182 provides the pump energy for VECSEL 170. A semiconductor saturable absorber mirror (SESAM) 184 provides passive mode-locking of VECSEL 170 and is separate from the gain region 174.
Today's passively mode-locked lasers use gain region and absorber materials that generally exhibit very similar saturation properties, so that rather different mode areas on the gain medium and the saturable absorber are required for mode locking. This is currently not achievable in monolithic structures.